The invention relates to thermal switches and particularly to miniature thermal switches utilizing a bimetallic element.
The prior art includes many thermal switches utilizing a circular bimetallic disc element. Such elements, at least in snap action switches, are bistable and change from a convex to a concave shape responsive to temperature change. The translation of such switch designs to a miniature size has not been wholly satisfactory because of difficult tolerance problems. More particularly, the physical movement of the bimetallic disc element due to temperature changes is often not sufficient to be compatible with production tolerances. Stated another way, for a specific temperature band there must be sufficient displacement to operate an electrical switch. The size of the bimetal element is constrained by the overall switch dimension requirements and the need for clearance for a cap or can support. Insufficient physical movement of the bimetal element due to temperature changes results in poor manufacturing yields and is typically overcome by imposing very tight tolerances on the switch components, which is costly.
The prior art includes at least one thermal switch made in Japan which utilizes an elongated bimetallic element in a snap action switch. The construction of that switch requires relatively complex assembly procedures including a requirement to tack weld an arm. The arm has an elongated bimetallic element fixed to the arm by means of tabs which extend from the arm to firmly engage the elongated bimetallic element. Thus, the manufacturing of such switches involve not only a tack weld but also a relatively difficult assembly involving the insertion of the bimetallic element into the tabs on the arms. The small size of the elements aggravates the difficulty of assembling the bimetallic element into the tabs of the arm and thus the assembly process for such switches is relatively complex. In addition the manufacturing of the individual components is also difficult.
The use of the full round bimetallic elements which are generally used requires that a can support be located around the perimeter of the disc seat. This reduces the possible size of the bimetallic element or requires a larger can.
It is an object of the invention to provide a bimetallic element for use in a miniature thermal switch which will have greater physical movement with a specific temperature change than was generally possible in bimetallic elements which have been generally utilized.
It is an object of the invention to provide a switch structure which will function with wider component tolerances in a given manufacturing lot and thus effectively result in a higher yield than has been previously been generally possible.
It is another object of the invention to maximize the size of the bimetallic element by locating the supports for the switch cap or cover at the side of and generally parallel to the bimetallic element, allowing the bimetallic element to be generally the same length as the switch base.
It is another object of the invention to provide apparatus which can be easily assembled and which minimizes the requirement for selective assembly to produce the final assembly.
It is an object of the invention to provide a thermal switch which is extremely compact.